The brain disease scrapie could be eradicated from the UK's entire population of sheep in a plan now under discussion.

Scientists have identified the genetics behind the susceptibility of sheep to the infection and believe animals could be selectively bred to make the national flock resistant.

The government is now consulting with sheep breeders and farmers about the desirability of the programme.

Although scrapie presents no direct threat to human health, researchers believe it would be better if the disease were eliminated.

Cascade of corruption

One theory suggests a strain of scrapie may have given rise to BSE, or mad cow disease, when cattle were fed on the rendered remains of infected sheep. And the meat from BSE-infected cattle is now widely accepted as the cause of vCJD in people.

Dr Matthew Baylis, a senior epidemiologist with the Institute of Animal Health, said scientists believed they now understood which alleles, or variations of a particular gene, opened sheep up to infection.

The alleles go by names such as ARR, ARQ and VRQ. The genes code for the so-called prion protein which is the agent thought responsible for transmission of diseases like scrapie.

When the prion becomes misshapen, a cascade of protein corruption takes place that progressively damages the brain.

Scientists have found that the VRQ allele offers the least resistance to corruption; ARR, on the other hand, offers the best resistance.

Three centuries

With this knowledge, it is being suggested that farmers have their rams tested to determine which animals should be used for breeding.

"The Ministry of Agriculture are proposing to undertake mass screening, at the government's expense, of British rams and ram-lambs so we can eradicate the VRQs and ARQs from the national flock," said Dr Baylis.

"Approximately 30% of sheep have the resistant genotype and with the assistance of breed societies and their members we would hope over 10 years to increase that percentage to 75%."

Dr Baylis said it might not be necessary to create 100% resistance to eradicate scrapie totally. The plan may also offer some protection against BSE infecting sheep.

Scrapie has been in the UK flock for 300 years. About 3% of the country's 50m sheep are thought to have the disease. But accurate figures are difficult to obtain because it is believed only about one in eight farmers actually report cases to the authorities.